1. Field of the Invention
This invention relates to an optical data recording and reproducing system using a semiconductor laser.
2. Background of the Invention
FIG. 1 shows a conventional drive circuit for a semiconductor laser (LD) in which a high frequency super-position technique is employed. FIG. 1 shows a high frequency oscillator 1 of several tens of megahertz (MHz) to several hundreds of megahertz. An automatic power control (APC) circuit 2 receives the detection signal of a monitor diode (MD) 5 adapted to detect the power of the laser beam emitted by the laser 4, to thereby make the power of the laser beam constant. The output currents of the high frequency oscillator 1 and the automatic power control circuit 2 are combined and applied through an LC circuit 3 to the semiconductor laser 4. The LC circuit 3 permits the application of the higher frequency signal to the semiconductor laser 4, but prevents the application of the high frequency signal to the automatic power control circuit 2.
Heretofore, by superposing a high frequency current as shown in FIG. 1, the laser is oscillated in a multi-mode or it is oscillated intermittently so that the oscillation of the laser is suspended when the laser beam is reflected back to the laser, whereby increase of laser noise is prevented.
FIG. 2 is a block diagram showing the APC circuit 2 shown in FIG. 1. A switching circuit in the APC circuit 2 is switched on or off by write data, which is input data processed by digital modulation, so that a constant current is added to change a light power from a low level to a high level, thereby producing pits.
The APC circuit controls the output power of the laser 4 which varies depending on the temperature. The auto power control of the APC circuit is carried out by comparing an output of a built-in monitor diode of the laser 4 with an objective value.
The write data M.sup.2 modulated at sync Ass'y modulator unit (not shown) is once latched by a write clock and transmitted to a laser power control unit. When the data is written, a large laser power is required. The large power is obtained by adding the constant current switched by the write data to a current at the reading time previous to the writing to operate the laser. When a disc has a constant angular velocity, linear velocities at inner and outer radial locations are different from each other. Optimum write power increases at an outer circumferential portion and, thus the value of a switching current is varied in proportion to a value of 4 bits of radial location data received from a drive CPU so as to change the power.
When the data is written, servo control is not carried out by the built-in monitor diode, since the inclination of operational characteristics (differential quantum efficiency) hardly changes even if the temperature changes. In practical use, since the retained power at reading time changes due to the drift of the monitor diode and the differential quantum efficiency also changes slightly due to the temperature, the temperature compensation of the added current is carried out.
Usually, a slow starter circuit having a large time constant is used to prevent an abrupt current from flowing at the ON time of the laser power. Mechanically, an inter lock switch is used so that if a disc cartridge is not completely set to a spindle motor, an electric power is not applied to the laser power control circuit.
FIG. 3 shows the laser beam power provided when the high frequency current is superposed. As is apparent from FIG. 3, in the case when the superposing high frequency current is increased, the peak value of the laser beam power is much higher than its average value during the recording operation. The optical disk responds to the average laser beam power but the degree of deterioration of the semiconductor laser LD is determined from the peak value of the recording laser beam power. Therefore, if deterioration of the semiconductor is sought to be minimized, then the average value of the laser beam power cannot be increased when the recording operation is carried out with the high frequency current being superposed. On the other hand, if the average value of the laser beam power is sought to be maximized, then the deterioration of the semiconductor laser is accelerated.